1. Field of the Invention
The present invention relates to a color signal processor, and more particularly to a color signal processing apparatus in an image pick-up apparatus which uses a single-plate color solid-state image pick-up device.
2. Related Background Art
In a single-plate color solid-state image pick-up device, the mosaic method and the stripe method have been proposed to construct a color filter. In the stripe method, respective color filter elements are arranged as vertical stripes. Because of the simple construction, it has been frequently used.
FIG. 4 shows a block diagram of a prior art signal processing circuit of a stripe type image pick-up apparatus. Numeral 10 denotes an image pick-up device having an RGB filter mounted on a front plane. It produces signals of three colors R, G and B. Amplifiers 12, 14 and 16 amplify the R signal, G signal and B signal, respectively, and the gains thereof are adjusted such that they are equal with respect to white color. A low pass filter (LPF) 18 limits bands of the signals R, G and B produced by the amplifiers 12, 14 and 16, and a gamma converter 20 effects a gamma conversion. The output of the gamma converter 20 is applied to a processing circuit 22 where two color differential signals R-Y.sub.L and B-Y.sub.L are produced by a matrix operation. Namely, EQU Y.sub.L =0.30R.sup..gamma. +0.59G.sup..gamma. +0.11B.sup..gamma.. . . (1) EQU R-Y.sub.L =0.70R.sup..gamma. +0.59G.sup..gamma. -0.11B.sup..gamma.. . . (2) EQU B-Y.sub.L =-0.30R.sup..gamma. -0.59G.sup..gamma. +0.89B.sup..gamma.. . . (3)
where R.sup..gamma., G.sup..gamma. and B.sup..gamma. are the three outputs from the gamma converter 20.
The R, G and B signals which have been gain-adjusted by the amplifiers 12, 14 and 16 are sequentially selected by a switch 24 by a read clock for each pixel to produce a high band luminance signal Y, which is band-limited by an LPF 26 and then gamma-converted by a gamma converter 28 to produce a final high band luminance signal Y.sub.SW.
In the prior art apparatus, when an image is reproduced in a receiver based on the luminance signal Y.sub.SW and the color differential signals R-Y.sub.L and B-Y.sub.L, the color reproduction is not of high fidelity because the luminance signal Y.sub.SW is not produced by the normal weighting defined by the NTSC standard (that is, the weighting of the formula (1)).
Several attempts to compensate it have been made. As will be explained below, the correction is non-linear and is not simple. Where the image changes relatively gently, the output Y.sub.H of the LPF 26 is generally a uniformly weighted average of the R, G and B signals: EQU Y.sub.H =(1/3)(R+G+B) . . . (4)
Accordingly, EQU Y.sub.SW =[(1/3) (R+G+B)].sup..gamma.. . . (5)
As is apparent from the comparison of the formula (5) and the formulas (2) and (3) , the formulas (2) and (3) are linear for R.sup..gamma., G.sup..gamma. and B.sup..gamma. but the formula (5) is not linear. As a result, the above correction is inevitably non-linear, which results in the increase in the circuit scale and the complexity.